Evolved UMTS Terrestrial Radio Access (E-UTRA) is expected to support several advanced antenna techniques. One advanced antenna technique is referred to as multiple input multiple output (MIMO). MIMO implies that both the base station and the UE has multiple antennas. There exist a variety of MIMO modes. Currently a number of MIMO modes are being evaluated such as Per Antenna Rate Control (PARC), selective PARC (S-PARC), transmit diversity, receiver diversity, D-TxAA (Double Transmit Antenna Array, an advanced version of Tx diversity).
The above mentioned MIMO modes provides different spatial processing which has the potential to contribute significantly to improve spectral efficiency, diversity, coverage, interference mitigation, etc. Each MIMO mode has a certain benefit. For example, PARC in principle can achieve high spectral efficiency by transmitting independent symbol streams; this means in a 2×2 PARC (2 transmit and 2 receive antennas), two independent data streams containing different information can be transmitted to the user equipment. On the other hand, receiver diversity (implying two receive antennas at the UE and one transmit antenna at the base station) increases link reliability by introducing redundancy in multiple dimensions at the receiver, but do not provide the spectral efficiency as PARC. This means receiver diversity would lead to good coverage but at the expense of lower spectral efficiency compared to PARC. In particular, spatial domain link adaptation for multi-antennas utilizes switching between different MIMO modes based on monitoring short-term characteristic of the radio channel.
Provided that the channel characteristics are known, a suitable MIMO mode may be selected. The selected criteria for selecting the MIMO mode may however differ. Based on measured transmission characteristic, different methods have been proposed to be used to determine the criteria for selecting the MIMO mode. In the prior art, the MIMI mode switching is only performed during an active connection between the UE (User Equipment) and the network. It should be noted that the UE is also referred to as terminal or mobile terminal.
During traffic inactivity the User Equipment (i.e. the mobile terminal) enters into a quasi-active state generally termed as idle state and utilizes the discontinuous reception (DRX) in order to save the power consumption of the UE battery. In the DRX mode the UE only monitors the network paging requests or performs certain types of measurements periodically. Due to user's mobility it is also important that the UE remains camped on the right cell. Therefore, the UE also measures the strength and/or quality of the downlink reference signals sent by the serving and target cells and re-selects the best cell. At cell reselection, the UE autonomously selects a new cell. However the reselection process can be partly influenced by the network, since the network can broadcast certain system parameters related to measurement thresholds, cell ranking etc. The cell reselection procedure should allow the network to identify the location of the UE on cell level or at least on some registration area level comprising of several cells. Thus in idle mode the network maintains the UE context, thus allowing the network to be able to locate the UE when sending the paging request. In case a new cell is reselected, the UE sends an update message to the network indicating the identity of the new cell. In addition the UE can also specify its capability, downlink measurement of serving and target cells etc. An appropriate cell reselection procedure is critically important for preventing new call blocking.
In the existing network, the network performs MIMO mode switching during the on going session. However, in idle state or any other low activity state (e.g. low Radio Resource Control (RRC) state) the UE may change cell and within the cell it may even change location. Thus a situation may arise when the UE does not start with the best possible MIMO mode and when it's RRC state changes from idle to active. This will lead to throughput loss until the network selects the correct MIMO mode. In some scenarios the connection can also be lost if the call is started with an inappropriate MIMO mode. It would therefore be advantageous to be able to exploit the benefits of mode switching also in idle state. However, the existing systems do not support a functionality whereby the UE and the network (e.g. associated Node B or RNC) are able to keep track of the best MIMO mode in idle state.